The present invention relates to a device for screening fiber suspensions, such as pulp, for dividing the fiber suspension into different length fractions or separating impurities and other fractions of the pulp undesired for the final product, such as coarse particles, undefibered material and poorly worked fibers.
It is known that variations in the concentration of a pulp are of decisive importance for the screening process. A decrease in concentration implies an increase of the hydraulic load on the screening means, i.e. the flow rate through the openings in the screening means increases. At concentrations below about 0.5% the total capacity becomes unacceptably low. An increase in the concentration implies instead an increase of the energy intensity required for breaking up the fiber network into individual fibers and make it fluid, i.e., so-called fluidization, which is a prerequisite for the screening process. The concentration, thus, defines a limit for effective utilization of the screen. Too high a concentration results in the flocks of the fiber suspension not being broken up, which implies that the screening process cannot continue.
In a conventional pressurized screen for pulp, the thickening along the length of the screening zone, from the inlet for unscreened pulp to the outlet, the reject chamber, for discharging concentrated impurities, is the physical problem, which limits the efficiency of the screen, with regard to both capacity and efficiency. The thickening means physically that the concentration of the fiber suspension increases from the inlet to the reject outlet along the surface of the screening means. Impurities are also concentrated from the inlet to the reject outlet. Increased concentration implies, that the strength of the fiber network increases considerably.
Since the rotating means of the screen rotates at an equal speed along the entire length of the screening zone, the energy supply is substantially constant from the inject end to the reject end of the screening means. This implies that the screening must start at too low a concentration at the beginning of the screening zone, in order to prevent the pulp concentration from rapidly becoming so high that a large portion of the screening zone acts as a thickener. Too high an energy intensity in relation to the pulp concentration implies, that the fiber suspension at the beginning of the screening zone has an unnecessarily high turbulence level, and thereby has a deteriorated separation selectivity. After a short zone under ideal conditions the pulp concentration is too high, the energy is no longer sufficient to break up the fiber network, and the final portion of the screening zone acts as a thickener. A high degree of thickening can also give rise to a braking effect, due to mechanically transferred force between the screening means and the rotating means. In other words, the thickening implies that the screen loses both efficiency and capacity.
One has succeeded in increasing the pulp concentration in certain modern pulp screens by placing inside the screening means a rotating means with pulsation creating wings, which yield an extended suction pulse, which, in turn, creates a vacuum on the outside of the pulsation wing, i.e. adjacent the screen means, in order to recover through the screening means a certain amount of the liquid which is lost by the thickening end in order to keep the screening means open. At the same time an overpresssure is created on the inside of the pulsation wing. The difference in pressure between the inside and the outside of the pulsation wing results in the rear edge of the wing as seen in the direction of rotation, a flow of pulp from the inside of the pulsation wing to its outside. Extended suction pulses through wide pulsation wings make it possible to increase the concentration in a screen, due to the fact that more liquid can be recovered. The screening means, however, is then subjected to very high loads. Problems can also arise with erosion on the pulsation wings. The stresses become especially high in the final portion of the screening zone, because the concentration of the pulp at that point is the greatest, and the pulp contains a great amount of impurities. Variations in the pulp concentration, dewatering properties or fiber length distribution affect the critical balance between network strength and energy supply. This results in one being forced to operate the screen with higher than optimum speed in order to manage the operability, even at normal process variations.
One object of the present invention is a screening device where the aforesaid problems can be reduced considerably in that the pressure difference between the inside and outside of the pulsation wing is reduced at the end of the screening zone.
This and other objects can now be realized by forming the pulsation wing at the end of the screening zone, namely at its lower portion, in such a way that its extension in the tangential direction substantially decreases gradually in the direction towards the reject outlet, and particularly where this decrease is created at the rear edge of the pulsation wing.
In accordance with the present invention, these and other objects are thus achieved by the invention of apparatus for screening fiber suspensions comprising a housing including an inlet for the fiber suspension, a reject outlet for removal of a reject portion of the fiber suspension, and an outlet for receipt of an accept portion of the fiber suspension, a screen member located within the housing, a rotor rotatably mounted in the screen member and forming a screening zone therebetween, the rotor including a plurality of pulsation wings mounted on the rotor, a reject chamber in the housing for collecting the reject portion of the fiber suspension which does not pass through the screen member and for supplying the reject portion to the reject outlet, and an accept chamber in the housing for collecting the accept portion of the fiber suspension which passes through the screen member and for supplying the accept portion to the outlet, each of the plurality of wing members including an inside face facing the rotor, an outside face facing the screen member, a leading edge facing the direction of rotation of the rotor, a trailing edge facing away from the direction of rotation of the rotor, an upper end and a lower portion including a lower end, the lower end facing the reject chamber, whereby a predetermined pressure difference is created between the outside face and the inside face of the at least one of the plurality of pulsation wings, the predetermined pressure difference decreasing in the lower portion of the at least one of the pulsation wings in the direction of the lower end thereof. In a preferred embodiment, the predetermined pressure difference is created between the outside face and the inside face of each of the plurality of pulsation wings, the predetermined pressure difference decreasing at the lower portion of the plurality of the pulsation wings in the direction of the lower ends thereof. Preferably, each of the plurality of pulsation wings includes a predetermined maximum extension in a tangential direction between the leading edge and the trailing edge, and wherein the lower portion of the plurality of pulsation wings where the predetermined pressure difference decreases comprises a predetermined axial portion of the plurality of the pulsation wings having a predetermined axial length comprising from about 0.5 to 2 times the predetermined maximum extension. In a preferred embodiment, the lower portion of the plurality of the pulsation wings where the predetermined pressure difference decreases comprises a predetermined axial portion of the plurality of the pulsation wings having a predetermined axial length comprising from about 1 to 1.5 times the predetermined maximum extension.
In accordance with one embodiment of the apparatus of the present invention, each of the plurality of the pulsation wings includes an extension in the tangential direction between the leading edge and the trailing edge, and wherein the extension gradually decreases at the lower portion of the plurality of the pulsation wings. In a preferred embodiment, the gradual decrease in the extension at the lower portion of the plurality of the pulsation wings comprises a reduction from the trailing edge of the plurality of the pulsation wings. More preferably, the plurality of the pulsation wings includes a predetermined maximum extension in the tangential direction, and a predetermined minimum extension in the tangential direction at the lower end of the plurality of the pulsation wings, wherein the predetermined minimum extension is at least one-fifth of the predetermined maximum extension. Most preferably, the predetermined minimum extension is at least one-fourth of the predetermined maximum extension.
In accordance with the present invention, a rotor has also been invented which is adapted to be disposed in a screening apparatus including a housing and a screen member located within the housing, the rotor comprising a plurality of pulsation wings, each of the plurality of pulsation wings including an inside face facing the rotor, an outside face adapted to face a screen member, a leading edge adapted to face in the direction of rotation of the rotor, a trailing edge adapted to face against the direction of rotation of the rotor, an upper end and a lower portion including a lower end, whereby a predetermined pressure difference can be created between the outside face and the inside face of at least one of the plurality of pulsation wings, the predetermined pressure difference decreasing at the lower portion of the at least one pulsation wing in the direction of the lower end thereof.
In a pulsation wing of conventional type, where the pressure difference between the inside and the outside is great even at the lower portion of the pulsation wing, a flow of pulp from the reject chamber to the outside of the pulsation wing is obtained. This is due to the fact that the edge of the pulsation wing toward the reject chamber, i.e. the lower edge, during rotation moves against a suspension which, on a relative basis, is essentially standing still in the reject chamber. This means physically that a flow from the reject chamber is more favorable than that in which the pulp flows over the lower edge of the pulsation wing from its inside to its outside, or through the screening means to the outside of the pulsation wing. The flow of pulp from the reject chamber to the screening zone contributes to a considerably deteriorated efficiency and capacity of the screening device, because this contributes strongly to a higher pulp concentration in the lower portion of the screening zone. The flow of pulp from the reject chamber to the screening zone also counteracts the downfeed of reject along the screening means to the reject chamber.
With a pulsation wing formed according to the present invention a substantially reduced flow of pulp from the reject chamber to the screening zone is obtained, compared to a conventional screening device.
The present invention also relates to a rotor for use in a screening device of the aforesaid kind.